Strokes/outlines that have gradients sometimes don't render properly

[adroitwhiz]

Expected Behavior

All SVG elements should appear on-stage the same way they do in the paint editor

Actual Behavior

Strokes with gradients, in some cases, appear single-color on the stage:

This is blocking scratchfoundation/scratch-paint#1004

Steps to Reproduce

1. Save gradientstroke.txt and change the file extension to .svg
2. Upload that file as a costume
3. Observe the difference between how the costume appears on-stage and how it appears in the paint editor

Operating System and Browser

All